Walnuts' Secret Defense Explored

With their rich taste and pleasing
crunchiness, it's no wonder that walnuts are one of America's favorite tree
nuts. But walnuts, like several other kinds of crops, are vulnerable to attack
by fungi called Aspergillus flavus and A. parasiticus. Both
species can produce a natural compound called aflatoxin, which is thought to be
carcinogenic.

Though inspections ensure that the walnuts that make their way from orchards
to the home are free of harmful levels of aflatoxin,
Agricultural Research Service scientists
in Albany, Calif., want to do more to combat the fungi. For example, they've
compared the A. flavus resistance of nearly a dozen leading kinds of
English walnuts--the kind most widely marketed in the United States--and two
species of black walnuts, which are less widely grown because their thicker
shells are harder to open.

Tulare's secret defense? It's gallic acid, found only in the nutmeat's thin
skin, or pellicle, according to Molyneux. Tulare walnuts contained
one-and-one-half to two times more gallic acid than, for instance, Chico
walnuts, the most Aspergillus-susceptible of the walnuts that the
researchers tested.

Earlier, scientists elsewhere had shown that gallic acid has antimicrobial
effects. But Molyneux's team is likely the first to show that a crop
susceptible to an aflatoxin-producing Aspergillus can actually prevent
the Aspergillus from producing aflatoxin.

The work, published in 2003, paved the way for current Albany studies to
discover how gallic acid undermines Aspergillus.

Read
more about the research in the March issue of Agricultural Research
magazine.